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# Light

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by

## Emma van Leeuwen

on 13 July 2015

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#### Transcript of Light

Light
generally travels in straight lines
3 things can happen when light hits a surface:
Specular reflection
(Smooth surface)
Diffuse reflection
(Rough surface)
Reflection
Mirrors
Secular reflection
http://www.acoustics.salford.ac.uk/schools/teacher/lesson3/flash/whiteboardcomplete.swf
Properties of an image
http://www.acoustics.salford.ac.uk/schools/teacher/lesson3/flash/whiteboardcomplete.swf
Optical fibers
Refraction
Light travels from air to water with an angle of incidence of 20°. What is the angle of refraction?
Example
Ray tracing
to predict properties of image
Plane mirror
Plane
Circular concave
Parabolic concave
4 standard rays
Ray // to principal axis reflected thru F
Ray thru F reflected // to principal
Ray at pole = plane mirror
Ray thru C back same path
* Ray tracing WS 1
Concave mirrors
Convex mirrors
The bending of light as it passes from one medium into another
due to it's change in speed
Snells law for any medium
speed of light in vacuum
speed of light in material
=
refractive index
of a material
How fast does light travel in water?
How fast does light travel in diamond?
Apparent depth
refractive index
real depth
apparent depth
=
The refractive index of water is 1.33.
The real depth of the fish is 2 m.
What is the depth the person perceives?
When the angle between the normal and a ray in fluid (with n=1.45) is increased beyond a certain critical angle, the light ray does not emerge from the fluid.

Calculate this critical angle in degrees.
Draw what happens to the light incident at angles greater than the critical angle.
Example
Lenses
Convex lens
Concave lens
Angled rays refracted thru a focal point somewhere along the focal plane
3 standard rays
Ray // to principal axis refracted thru f
Ray thru F reflected // to principal
Ray thru center keeps going
* Ray tracing WS 2
*intersection of these = image location
*intersection of these = image location
Example:
u = 2f
u = f
u < f
Dispersion
Electromagnetic wave spectrum
Electromagnetic model of light
Polarisation of light waves
Recap
3 things can happen when light hits a surface:
2 types of reflection
A person 1.6m tall, looks into a plane mirror and wants to be able to see their entire image. How large must the mirror be?
A 0.8 m B 1.6 m C 2.4 m D 3.2 m

The person walks away from the mirror. How does the image change in the mirror?
A they see more of themselves
B they see the same amount of themselves
C they see less of themselves
D how the image changes depends on the size of the mirror.
Question 5
Question 6
When a blue torch and a yellow torch are both shone onto green paper, the paper will appear
A. White B. Cyan C. Blue D. Green

Question 4
Law of reflection
The law of refection applies to:
A All surfaces
B Specular reflection only
C Plane mirrors but not concave or convex mirrors
Question 7
Refraction
The bending of light as it passes from one medium into another
Snells law for any medium
If n1 < n2 the light it is:
A Bent towards the normal
B Bent away from the normal
C Not bent
Question 8
Electromagnetic waves are best describes as:
A Longitudinal waves
B Transverse waves
C A mixture of transverse and Longitudinal waves
D Neither, they are just 'rays' of energy
Question 1
Total internal reflection
TIR can only occur (select two answers):
A when light tries to pass from an optically dense medium to an optically light one.
B when light tries to pass from an optically light medium to an optically dense one.
C when light tries to pass from air to the water.
D when light tries to pass from water to the air.

Question 9
What happens to light incident at angles greater than the critical angle?
Question 10
Question 2
Which one or more of the arrows labeled A to F best describes the size of the amplitude?

Which best describes the wavelength?
Question 3
Electromagnetic waves are best describes as:
A Longitudinal waves
B Transverse waves
C A mixture of transverse and Longitudinal waves
D Neither, they are just 'rays' of energy
Question 1
Question 2
Which one or more of the arrows labeled A to F best describes the size of the amplitude?
D

The wavelength?
C, E
Question 3
When a blue torch and a yellow torch are both shone onto green paper, the paper will appear
A. White B. Cyan C. Blue
D. Green

Question 4
A person 1.6m tall, looks into a plane mirror and wants to be able to see their entire image. How large must the mirror be?
A 0.8 m
B 1.6 m C 2.4 m D 3.2 m
Question 5
Question 6
The law of refection applies to:
A All surfaces
B Specular reflection only
C Plane mirrors but not concave or convex mirrors
Question 7
If n1 < n2 the light it is:
A Bent towards the normal
B Bent away from the normal
C Not bent
Question 8
TIR can only occur (select two answers):
A when light tries to pass from an optically dense medium to an optically light one.
B when light tries to pass from an optically light medium to an optically dense one.
C when light tries to pass from air to the water.
D when light tries to pass from water to the air.
Question 9
What happens to light incident at angles greater than the critical angle?
It is reflected by the law of reflection (angle incidence = angle reflection)
Question 10
The person walks away from the mirror. How does the image change in the mirror?
A they see more of themselves
B they see the same amount of themselves
C they see less of themselves
D how the image changes depends on the size of the mirror.
Opaque
Translucent
Transparent
Amount transmitted:
Normal
i
r
i = r
Law of reflection:
Law of reflection holds for all mirrors
Nature
Size
Location
Orientation
virtual
real
Upright
Inverted
Parallel rays converge on focal point
Convex mirrors
Convex lens
Lens Prac
V
u
move card until image becomes clear
measure v, calculate F and M

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n
=
c
v
_
v = c/n = 2.26E8 m/s
v = c/n = 1.24E8 m/s
Refractive index
14.9
o
sin(x) = n1/n2 sin(20)
= 1/1.33 sin(20)
x =
Total internal reflection
Light refracted at 90 degrees
Critical angle
Bends towards normal as it slows down
Bends away from normal if speeding up
Bends towards normal if slowing down
Recall: light bends away from the normal when speeding up....
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apparent depth = real/n
= 2/1.33
= 1.5 m
Example
Mirage
sin ( ) =
n
n
2
1
_
_
c
43.6
o
sin(x) = n2/n1
= 1/1.45
x =
N
N
Qs 1-5, 11-12
Chapter 9: refracting light
Refraction
The bending of light as is passes from one medium into another
(behind mirror)
(in front of mirror)
4 principal rays
Ray // to principal axis reflected thru f
Ray thru f reflected // to principal
Ray hitting pole reflects at same angle opp side
intersection of any 2 of these = image location
Ray thru C reflects back the way it came
C = 2f
Normal
i
r
i = r
Law of reflection always true:
Refraction
c
<
c
=
c
>
Total internal reflection
f =
uv
u+v
_
_
_
_
M =
v
u
_
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Full transcript